Any material, whether natural or synthetic must exhibit satisfactory resistance to degradation under conditions of use, if products made from the materials are to find a lasting market. A lack of satisfactory resistance to degradation usually manifests itself as a partial or total loss of structural integrity, a darkening or discoloration of the product, a loss of flexibility or resilience, or a combination of the above phenomena. These phenonmena are promoted or catalyzed by air (oxygen), heat and light, and are particularly susceptible to autooxidation at elevated temperatures in the presence of oxygen.
To protect organic materials, ingredients which can be collectively called stabilizers are admixed with the materials to prevent or inhibit degradation. These stabilizers work in diverse and complex ways, such that a compound which stabilizes against oxygen degradation in one type of material may be relatively inactive in another type of material. Thus compounds which are stabilizers are further classed as antioxidants, antiozonants, heat stabilizers and ultraviolet (UV) light stabilizers, depending upon what type of activity and stabilization they best demonstrate. In many cases, to obtain optimum protection, a mixture of compounds, each specifically selected to afford maximum protection against a certain type of degradation, is often used. In some instances stabilizers are deliberately chosen to counter the adverse effects of a plasticizer which, though highly effective as a plasticizer, tends to accelerate oxygen and heat degradation. In other words, the plasticized material is more susceptible to degradation than if no plasticizer was added. As a general empirical rule, it is found that plasticizers are marginally effective as stabilizers, and stabilizers are marginally effective as plasticizers, it being more likely that a compound with desirable stabilizer properties has undersirable plasticizer properties, and vice versa.
The stabilization of rubber, and particularly synthetic "natural rubber", is essential for its proper functioning and long life. Although most antioxidants give good protection as stabilizers, not all stabilizers give satisfactory antioxidant activity (Encyclopedia of Polymer Science and Technology, Vol. 12, btm p 267, Interscience Publishers, New York, 1970). The compounds of this invention are primarily antioxidants though they exhibit other desirable stabilizing properties. They are particularly useful in synthetic natural rubber (SNR), specifically to prevent the staining of whitewall tires on passenger car tires. They are also particularly effective in injection-molded polypropylene articles such as the rotors of washing machines, synthetic resinous floor mats for automobiles, hose for oil coolers, fuel hoses, hydraulic brake lines, and vividly pigmented garden hose. The compounds are mainly used as a primary antioxidant, i.e. as the sole antioxidant, but if desired, may be combined with a secondary antioxidant which serves to enhance the stabilizing performance of the primary antioxidant. When used in combination with a secondary antioxidant, the stabilizing effect achieved is sometimes synergistic and the performance of the combination substantially exceeds the sum total of the performances exhibited by the individual antioxidant components.
The time-tested rubber antioxidants chemically classed as amines and phenols and their respective derivatives are still being used, but newer antioxidants combine a hindered phenol group with another group containing sulfides, triazine, phosphates, phosphites, etc. with the expectation that the active material produced will combine the advantages of its two or more component stabilizing moieties.
The compounds of this invention do not belong to any well-recognized chemical class of antioxidants. They are polymers having a vinyl aromatic backbone to which is attached at least one alkyl substituted 1,2-dihydroquinoline moiety.
As is well-known to those skilled in the art, the effectiveness of an antioxidant is predicated upon the oxidizable material in which the antioxidant is used. Thus, though antioxidants are used in plastics, elastomers, petroleum products, synthetic lubricants, food products, paints, soaps and cosmetics, it is seldom that the same type of antioxidant will be useful in a plastic or elastomer, and a petroleum or synthetic lubricant. Yet the compounds of this invention provide just such a multifunctional purpose, being useful in several synthetic resinous materials including plastics, elastomers and particularly conjugated diene polymers.
Though a wide variety of antioxidants is effective in a white-wall of a tire, each has certain shortcomings. For example, Age Rite Resin D*, a polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (hereinafter "polyTMDQ" for brevity), commercially available from R. T. Vanderbilt Co. (see Encylcopedia of Polymer Science and Technology, Vol. 2, p 190, published by Interscience Publishers, John Wiley & Sons, Inc. 1965), has a tendency to stain the white-wall of a vehicle's tire. Unless one makes and sells white-walled tires, this consideration is of minor importance. Nonetheless, it should not be surprising that an excellent antioxidant should fall a little short with respect to one or more of the eight basic attributes of a quintessential antioxidant (see id., supra, p 185). Since there are known antioxidants notably less prone to stain white-walls, it is of especial interest that I have passed over these known antioxidants as a starting point for the production of non-staining antioxidants, in favor of TMDQ which is known to stain. It is of even greater interest that combining a vinyl aromatic resin, which by itself is devoid of any stabilizing effects in rubber, with TMDQ, should result in a non-staining stabilizer. In this regard it is to be noted that the alkylation of benzene or monomeric styrene with TMDQ yields a reaction product which has no measurable stabilizing effect in polypropylene. FNT *U.S. Trademark
Since there are no guidelines, and certainly no rules for tailoring an antioxidant to be non-staining, one considering the possibilities of a likely combination of an aromatic moiety with a bicyclo moiety is not impelled to choose an alkylation reaction because these moieties are known to be difficult to alkylate. It is also difficult to choose any one of a class of alkylation catalysts from among the classes of alkylation catalysts, each of which catalysts may have a wide range of effectiveness in a particular reaction. Moreover it is known that one cannot predict the effectiveness of acid-acting Friedel-Crafts catalysts for alkylation reactions, much less the effectiveness of a specific Friedel-Crafts catalyst, in a particular, desirable alkylation reaction. It is even less likely that one can predict the alkylation of a polyvinyl aromatic resin with a substituted 1,2-dihydroquinoline, assuming one was desirous of doing so.
Now, it has long been known to use certain vinyl aromatic monomers in a reaction mixture in which a condensation of a bisphenol, such as 2,2-bis(4-hydroxyphenyl)propane, with an olefin yields an alkylated bisphenol (see U.S. Pat. No. 3,022,269 to Jansen, Jacob E. and Kehe, Henry J.). The purpose of introducing from about 5 to about 25 parts of styrene to 100 parts of isobutylene into the alkylated reaction product was to overcome a tendency towards crystallization of the product.
It has been known for even longer (see U.S. Pat. No. 2,400,500 to Gibbs, Carlin F.), that alkyl-substituted, 1,2-dihydroquinolines may be condensed with diarylamines in the presence of catalysts of the Friedel-Crafts type, so that the diarylamine adds to the double bond of the dihydroquinoline to form a substituted tetrahydroquinoline which is a good antioxidant. But a diarylamine has very little in common with a polyvinyl aromatic resin such as polystyrene, a dissimilarity emphasized by the fact that the reaction of TMDQ with a diarylamine proceeds relatively easily with any Friedel-Crafts type catalyst, while the reaction of TMDQ with polystyrene (say) proceeds only with at least a stoichiometric amount of a metal halide Friedel-Crafts type catalyst in the presence of a suitable solvent for the polymer.
From the foregoing and numerous other references, it will now be evident that in the far-flung classification of stabilizers which are unpredictable as to the desirable effects of certain moieties and their off-setting drawbacks, there is no reason to expect that a polystyrene moiety, which itself has no desirable antioxidant properties whatsoever, should inculcate any desirable attributes to an antioxidant, least of all, any non-blooming and non-staining attributes. Further, I know of no basis upon which to predicate the alkylation of polyvinyl aromatic resin with any substituted 1,2-dihydroquinoline, nor any reason to expect that if such an alkylated reaction product were made, there would enure to it excellent antioxidant properties combined with an outstanding (a) lack of bloom to the surface, and (b) a disinclination to stain substances with which it comes into contact.